The present invention relates to a pneumatic tire, more specifically, to a pneumatic tire suitable for running on an ice and snow road, which is capable of improving ice performance without lowering driving stability on a dry road surface, and of suppressing occurrence of uneven wear.
Generally, a pneumatic tire used on the ice and snow road is constituted in such a manner that a large number of blocks are defined on a tread surface by main grooves extending in a circumference direction of the tire and side grooves extending in a width direction of the tire, and that sipes are provided on contact surfaces of the blocks. Generally, the sipes extend straight in a depth direction of the tire. A groove area is secured by the main grooves and the side grooves, thus braking and driving performance of the tire on snow (snow performance) is exerted. Meanwhile, braking and driving performance of the tire on ice (ice performance) is secured by edge effects of the blocks and the sipes.
Incidentally, in recent years, a xe2x80x9cMirror Bahn Phenomenonxe2x80x9d that a road surface is frozen in a cold district in winter, becomes smooth like a mirror plane and tends to be slippery has frequently occurred, which is one of causes causing a serious accident. Therefore, in the pneumatic tire for the ice and snow road, further improvement for the ice performance has been required.
However, when the number of sipes is simply increased in order to further improve the ice performance, rigidity of the blocks has been lowered, thus the driving stability on the dry road surface has been lowered. Furthermore, there has been a problem that uneven wear occurs.
The object of the present invention is to provide a pneumatic tire capable of improving ice performance without lowering driving stability on a dry road surface, and of suppressing occurrence of the uneven wear.
The present invention achieving the foregoing object is a pneumatic tire provided with sipes extending in zigzag on tread surfaces, characterized in that both wall surfaces of each of the sipes, the wall surfaces facing to each other, are inclined with respect to a diameter direction of the tire, and constituted of triangular wall surfaces, each having two sides expanding toward a sipe bottom with a bending point of a zigzag of the sipe as an apex and of inverse triangular wall surfaces, each having two sides narrowed toward the sipe bottom with one side of the zigzag of the sipe as a base, and a pair of the triangular wall surfaces adjacent to each other and a pair of the inverse triangular wall surfaces adjacent to each other are alternately arranged.
As described above, the both zigzag wall surfaces of the sipe, which face to each other, are inclined with respect to the diameter direction of the tire. Thus, an area of the entire wall surface of the sipe can be widened more than that of a conventional sipe extending straight in a depth direction of the tire. Accordingly, a contact pressure between the wall surfaces of the sipe contacting with each other during shear deformation of the block and rib formed on a tread surfaces at the time of the braking and driving of the tire can be increased.
Moreover, the both wall surfaces of the sipe, which face to each other, are constituted of triangular wall surfaces, each having two sides expanding toward a sipe bottom with a bending point of a zigzag of the sipe as an apex and of inverse triangular wall surfaces, each having two sides narrowed toward the sipe bottom with one side of the zigzag of the sipe as a base, and a pair of the triangular wall surfaces adjacent to each other and a pair of the inverse triangular wall surfaces adjacent to each other are alternately arranged. Thus, the zigzag wall surfaces of the sipe can be made in contact with each other just by compression and deformation of the block or the rib while allowing forces alternately inverse to act on the zigzag bending portions of the sipe. Accordingly, it is made possible to further increase the contact pressure between the wall surfaces of the sipe, which contact with each other, during the braking and the driving of the tire. Therefore, since the blocks or the ribs can be formed so as to be hard to fall, lowering of the block rigidity due to provision of the sipe is not substantially brought. Accordingly, the driving stability on a dry road surface is not lowered. Moreover, since the rigidity of block or rib is high and not partially varied, it is made possible to suppress the occurrence of the uneven wear.
Accordingly, during the braking and the driving of the tire, it is made possible to increase the number of the sipes while maintaining the rigidity of block or rib, thus the ice performance can be improved by the edge effect of the increased sipes.